In a wireless communication system having a wireless base station (BS) and a wireless terminal (for example, a mobile station) (MS), the MS can communicate with another MS via the BS.
In such a wireless communication system, there is generally provided QoS (Quality of Service) that reserves a bandwidth for a specific communication and assures a certain transmission speed.
The QoS realizes a communication based on priority for each user according to information (contract information, etc.) about the user possessing the MS, for example. The Qos can provide communication of higher quality or wider bandwidth to a user having higher priority than a user having lower priority.
In a wireless communication system in conformity to IEEE802.16e standard such as WiMAX (Worldwide Interoperability for Microwave Access) based on OFDM (Orthogonal Frequency Division Multiplexing) (or OFDMA: Orthogonal Frequency Division Multiple Access) technique that has been researched and developed in these years, the MS transmits a specific signal called a CDMA code when opening a communication with a BS, and receives a response signal to the CDMA code from the BS which includes transmission permission to transmit an allocation request for a wireless resource (frequency bandwidth, spread code, etc.) for data transmission.
Now, an example of operation of the whole wireless communication system aforementioned will be described with reference to FIG. 12. FIG. 12 is a sequence diagram illustrating an operation at the time of start of a communication in the above wireless communication system.
Before starting a communication with the BS, the MS transmits a CDMA code to the BS in order to obtain a right to transmit an allocation request for the wireless resource (step S100).
When normally receiving the CDMA code from the MS, the BS determines whether to permit the MS to transmit the wireless allocation request. This determination operation is based on the state of vacancy of the wireless resource or the like, for example.
When determining to permit the MS to transmit the allocation request, the BS transmits a message (CDMA Allocation IE (Information Element)) signifying that the BS permits the MS to transmit the allocation request as a response signal to the CDMA code thereby gives a right of transmission of the wireless resource allocation request signal to the MS (step S110).
When receiving the response signal, the MS transmits, to the BS, a wireless resource allocation request signal (Bandwidth Request) for the uplink (UL) communication (step S120).
When receiving the allocation request signal, the BS determines whether to allocate the requested wireless resource to the MS. This determination operation is based on the state of vacancy of the wireless resource or the like, for example.
When determining to allocate the wireless resource requested by the allocation request signal, the BS allocates the wireless resource requested by the allocation request signal to the MS, and transmits an allocation permission signal (UL Allocation) to the MS (step S130).
The MS finally receives the allocation permission signal, thereby initiating transmission in the UL (data transmission in the UL) with the use of the wireless resource allocated by the BS (step S140).
In the above wireless communication system, the MS needs that the BS accepts the CDMA code in order to transmit data to the BS, as stated above.
Now, a wireless frame used for transmission of the CDMA code will be described with reference to FIG. 13. FIG. 13 is a schematic diagram illustrating an example of wireless frame used in the above wireless communication system. This wireless frame is in conformity with the OFDM (or OFDMA) system, in particular, IEEE802.16e standard, used in WiMAX.
As illustrated in FIG. 13, the wireless frame is configured with a downlink (DL) subframe and an uplink (UL) subframe having a plurality of frequency channels (in the vertical direction on the paper) which are time-division-multiplexed (in the horizontal direction on the paper).
In the downlink subframe, there are defined a preamble field (region) 100 into which synchronization information on the wireless frame is inserted, a header region including FCH field into which a frame control header (FCH) is inserted, MAP information field into which MAP information (DL-MAP, UL-MAP) including allocation information on a burst region (defined by the frequency and symbol time) used for the communication by the MS is inserted, and another region (burst region).
On the other hand, the uplink subframe has a CDMA band width request region 200 and an other region (burst region, transmission region for MSs). The CDMA code is transmitted from the MS to the BS with the use of the CDMA bandwidth request region 200. Namely, the CDMA bandwidth request region 200 is defined as a region (common transmission region, common channel) common to a plurality of MSs.
Since the CDMA bandwidth request region 200 is common to a plurality of MSs (that is, MSs connecting to the BS) having been subjected to the negotiation process (user authentication process, etc.) required when the MS receives data from the BS, each of the plural MSs randomly selects one of plural CDMA codes, and transmits the CDMA code at a random frequency and timing (symbol time) with the use of the CDMA bandwidth request region 200.
When plural MSs transmit the identical CDMA codes to the BS at the same frequency and timing (symbol time) in the CDMA bandwidth request region 200, collision of the CDMA codes occurs, which prevents the BS from normally receiving the CDMA codes from the plural MSs.
As a result, the MS cannot receive a response signal (CDMA Allocation IE message) from the BS. Further, when the MS cannot receive the response signal from the BS within a predetermined period of time, the MS determines that the BS was not able to normally receive the CDMA code, and performs a control to select a CDMA code and re-transmit the CDMA code.
In the meantime, there are patent documents 1 to 3 as the related techniques. The technique disclosed in the patent document 1 defines two types of access control fields of CFP (Contention Free Period) and CP (Contention Period) in a wireless LAN (Local Area Network), and uses either CFP or CP according to a parameter for securing the minimum value of the CP in order to secure a constant CP at any time.
The technique disclosed in patent document 2 defines access control fields for CFP and CP so as to attain efficient use of the wireless resource and a wireless access control method applicable to multi hop communication. The technique disclosed in patent document 3 defines a contention mode and a poling mode in a communication system having a wire transmission path, and allocates an idle channel.
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-153558
Patent Document 2: Japanese Laid-open Patent Publication No. 2006-5653
Patent Document 3: Japanese Laid-open Patent Publication No. 2002-368764